:mod:`imp` --- Access the :keyword:`import` internals
=====================================================
.. module:: imp
:synopsis: Access the implementation of the import statement.
.. index:: statement: import
This module provides an interface to the mechanisms used to implement the
:keyword:`import` statement. It defines the following constants and functions:
.. function:: get_magic()
.. index:: pair: file; byte-code
Return the magic string value used to recognize byte-compiled code files
(:file:`.pyc` files). (This value may be different for each Python version.)
.. function:: get_suffixes()
Return a list of 3-element tuples, each describing a particular type of
module. Each triple has the form ``(suffix, mode, type)``, where *suffix* is
a string to be appended to the module name to form the filename to search
for, *mode* is the mode string to pass to the built-in :func:`open` function
to open the file (this can be ``'r'`` for text files or ``'rb'`` for binary
files), and *type* is the file type, which has one of the values
:const:`PY_SOURCE`, :const:`PY_COMPILED`, or :const:`C_EXTENSION`, described
below.
.. function:: find_module(name[, path])
Try to find the module *name*. If *path* is omitted or ``None``, the list of
directory names given by ``sys.path`` is searched, but first a few special
places are searched: the function tries to find a built-in module with the
given name (:const:`C_BUILTIN`), then a frozen module (:const:`PY_FROZEN`),
and on some systems some other places are looked in as well (on Windows, it
looks in the registry which may point to a specific file).
Otherwise, *path* must be a list of directory names; each directory is
searched for files with any of the suffixes returned by :func:`get_suffixes`
above. Invalid names in the list are silently ignored (but all list items
must be strings).
If search is successful, the return value is a 3-element tuple ``(file,
pathname, description)``:
*file* is an open :term:`file object` positioned at the beginning, *pathname*
is the pathname of the file found, and *description* is a 3-element tuple as
contained in the list returned by :func:`get_suffixes` describing the kind of
module found.
If the module does not live in a file, the returned *file* is ``None``,
*pathname* is the empty string, and the *description* tuple contains empty
strings for its suffix and mode; the module type is indicated as given in
parentheses above. If the search is unsuccessful, :exc:`ImportError` is
raised. Other exceptions indicate problems with the arguments or
environment.
If the module is a package, *file* is ``None``, *pathname* is the package
path and the last item in the *description* tuple is :const:`PKG_DIRECTORY`.
This function does not handle hierarchical module names (names containing
dots). In order to find *P*.*M*, that is, submodule *M* of package *P*, use
:func:`find_module` and :func:`load_module` to find and load package *P*, and
then use :func:`find_module` with the *path* argument set to ``P.__path__``.
When *P* itself has a dotted name, apply this recipe recursively.
.. function:: load_module(name, file, pathname, description)
Load a module that was previously found by :func:`find_module` (or by an
otherwise conducted search yielding compatible results). This function does
more than importing the module: if the module was already imported, it will
reload the module! The *name* argument indicates the full
module name (including the package name, if this is a submodule of a
package). The *file* argument is an open file, and *pathname* is the
corresponding file name; these can be ``None`` and ``''``, respectively, when
the module is a package or not being loaded from a file. The *description*
argument is a tuple, as would be returned by :func:`get_suffixes`, describing
what kind of module must be loaded.
If the load is successful, the return value is the module object; otherwise,
an exception (usually :exc:`ImportError`) is raised.
**Important:** the caller is responsible for closing the *file* argument, if
it was not ``None``, even when an exception is raised. This is best done
using a :keyword:`try` ... :keyword:`finally` statement.
.. function:: new_module(name)
Return a new empty module object called *name*. This object is *not* inserted
in ``sys.modules``.
.. function:: lock_held()
Return ``True`` if the import lock is currently held, else ``False``. On
platforms without threads, always return ``False``.
On platforms with threads, a thread executing an import holds an internal lock
until the import is complete. This lock blocks other threads from doing an
import until the original import completes, which in turn prevents other threads
from seeing incomplete module objects constructed by the original thread while
in the process of completing its import (and the imports, if any, triggered by
that).
.. function:: acquire_lock()
Acquire the interpreter's import lock for the current thread. This lock should
be used by import hooks to ensure thread-safety when importing modules.
Once a thread has acquired the import lock, the same thread may acquire it
again without blocking; the thread must release it once for each time it has
acquired it.
On platforms without threads, this function does nothing.
.. function:: release_lock()
Release the interpreter's import lock. On platforms without threads, this
function does nothing.
.. function:: reload(module)
Reload a previously imported *module*. The argument must be a module object, so
it must have been successfully imported before. This is useful if you have
edited the module source file using an external editor and want to try out the
new version without leaving the Python interpreter. The return value is the
module object (the same as the *module* argument).
When ``reload(module)`` is executed:
* Python modules' code is recompiled and the module-level code reexecuted,
defining a new set of objects which are bound to names in the module's
dictionary. The ``init`` function of extension modules is not called a second
time.
* As with all other objects in Python the old objects are only reclaimed after
their reference counts drop to zero.
* The names in the module namespace are updated to point to any new or changed
objects.
* Other references to the old objects (such as names external to the module) are
not rebound to refer to the new objects and must be updated in each namespace
where they occur if that is desired.
There are a number of other caveats:
If a module is syntactically correct but its initialization fails, the first
:keyword:`import` statement for it does not bind its name locally, but does
store a (partially initialized) module object in ``sys.modules``. To reload the
module you must first :keyword:`import` it again (this will bind the name to the
partially initialized module object) before you can :func:`reload` it.
When a module is reloaded, its dictionary (containing the module's global
variables) is retained. Redefinitions of names will override the old
definitions, so this is generally not a problem. If the new version of a module
does not define a name that was defined by the old version, the old definition
remains. This feature can be used to the module's advantage if it maintains a
global table or cache of objects --- with a :keyword:`try` statement it can test
for the table's presence and skip its initialization if desired::
try:
cache
except NameError:
cache = {}
It is legal though generally not very useful to reload built-in or dynamically
loaded modules, except for :mod:`sys`, :mod:`__main__` and :mod:`__builtin__`.
In many cases, however, extension modules are not designed to be initialized
more than once, and may fail in arbitrary ways when reloaded.
If a module imports objects from another module using :keyword:`from` ...
:keyword:`import` ..., calling :func:`reload` for the other module does not
redefine the objects imported from it --- one way around this is to re-execute
the :keyword:`from` statement, another is to use :keyword:`import` and qualified
names (*module*.*name*) instead.
If a module instantiates instances of a class, reloading the module that defines
the class does not affect the method definitions of the instances --- they
continue to use the old class definition. The same is true for derived classes.
The following constants with integer values, defined in this module, are used to
indicate the search result of :func:`find_module`.
.. data:: PY_SOURCE
The module was found as a source file.
.. data:: PY_COMPILED
The module was found as a compiled code object file.
.. data:: C_EXTENSION
The module was found as dynamically loadable shared library.
.. data:: PKG_DIRECTORY
The module was found as a package directory.
.. data:: C_BUILTIN
The module was found as a built-in module.
.. data:: PY_FROZEN
The module was found as a frozen module (see :func:`init_frozen`).
.. class:: NullImporter(path_string)
The :class:`NullImporter` type is a :pep:`302` import hook that handles
non-directory path strings by failing to find any modules. Calling this type
with an existing directory or empty string raises :exc:`ImportError`.
Otherwise, a :class:`NullImporter` instance is returned.
Python adds instances of this type to ``sys.path_importer_cache`` for any path
entries that are not directories and are not handled by any other path hooks on
``sys.path_hooks``. Instances have only one method:
.. method:: NullImporter.find_module(fullname [, path])
This method always returns ``None``, indicating that the requested module could
not be found.
.. _examples-imp:
Examples
--------
The following function emulates what was the standard import statement up to
Python 1.4 (no hierarchical module names). (This *implementation* wouldn't work
in that version, since :func:`find_module` has been extended and
:func:`load_module` has been added in 1.4.) ::
import imp
import sys
def __import__(name, globals=None, locals=None, fromlist=None):
# Fast path: see if the module has already been imported.
try:
return sys.modules[name]
except KeyError:
pass
# If any of the following calls raises an exception,
# there's a problem we can't handle -- let the caller handle it.
fp, pathname, description = imp.find_module(name)
try:
return imp.load_module(name, fp, pathname, description)
finally:
# Since we may exit via an exception, close fp explicitly.
if fp:
fp.close()
.. index:: module: knee
A more complete example that implements hierarchical module names and includes a
:func:`reload` function can be found in the module :mod:`knee`. The :mod:`knee`
module can be found in :file:`Demo/imputil/` in the Python source distribution.